High voltage electric power cables are used to transmit electric power with medium or high voltage. The cables normally comprise a conductor and a polymeric insulation system that surrounds the conductor. Electric power cables that can be buried into the ground are called land cables. Electric power cables that can be buried into a sea bed which can freely extend between two fixing points in sea water are called submarine, sea water or underwater power cables. Underwater power cables are used today in an increasing amount due to the increased need of power transfer from for example offshore energy sources, including offshore renewable energy plants, such as wind power plants. Also the length for power transfer cables is increasing since there is a need to interconnect different regional electrical transmission networks to allow global trading of energy. Areas where energy is on the other hand needed and on the other hand produced may also be remote from each other which further increases a need for safe power transfer.
In order to meet the demands for safe power transfer, protection of the insulation systems need to be of high quality to ensure correct electrical and mechanical behaviour during the transmission of electric power. To electrically insulate the conductor, an insulation system including semi-conducting and insulating polymeric layers is arranged to surround the conductor. Unless the power cables are appropriately insulated, significant leakage currents will flow in the radial direction of the cables, from the conductor to the surrounding grounded screen. To protect the insulation system and the conductor of the high voltage power cable, metal laminates or e.g. extruded lead sheath can be provided around the insulation system as a water barrier. A polymer jacket may then be applied on top of the laminate or sheath. High voltage cables are often also include water blocking tape (WBT) also known as swelling tape that surrounds the insulation system of the cable. The water blocking tape is mainly provided to stop longitudinal spreading of water in the cable in case the cable is damaged. The water blocking tape includes a hygroscopic, i.e. water absorbing material that swells in contact with water and prevents leakage of water in longitudinal (and radial) direction of the cable so that the damaged portion of the cable can be minimized.
The use of water blocking materials have been discussed in the prior art. Publication “IEEE Transactions of Industry Applications, Vol. 29, No. 5, September/October 1992; W. F. (Buddy) Power, Jr; “An overview of Water-Resistant Cable designs” discloses the use of plastic compounds and water-absorbing materials within conductor strands to achieve longitudinal water blocking in cables. The document also discloses the use of encapsulating jackets and metallic laminated tapes as water blocking materials.
However, even though satisfactory water blocking characteristics have been achieved by the use of e.g. water blocking tapes, it has been noticed that the water blocking tapes can have a negative impact on the electrical performance of high voltage direct current (HVDC) power cables. Substances originating from the water blocking tapes can migrate into the insulation giving rise to the electrical DC conductivity in the insulation. This will increase the dielectric losses in the cable and thus increase the risk for thermal runaway. Thus, even though there are known solutions for water-resistant cable designs there is still a need to improve the known designs.